1. Field of the Invention
The present invention relates to surfaces used to reflect light, and particularly to highly light reflectant surfaces that provide even diffusion of light energy from their surfaces.
2. Description of Related Art
Special light reflectant surfaces are used in a variety of applications requiring light energy to be close to completely reflected while providing an even distribution of light from the surface. While good mirrored surfaces can provide nearly perfect reflectivity of visible light, the light energy exiting these surfaces does so only at an angle equal to the incident angle of light contact. For many applications it is important that light be reflected with an even distribution of light from the surface. This latter property is referred to as diffuse or "lambertian" reflectance.
For instance, projection screens, such as those used for slide or motion picture presentations, must provide both high reflectivity and a light diffusion/distribution over a sufficiently wide field so as to provide a clear image to most of an audience. Many reflectant screens employ a coating of glass beads or similar material as a reflectant aid to provide excellent reflectivity over a defined projection field (e.g., approximately 20.degree. from a center line), with significantly diminished reflectivity outside of the defined projection field. These screens provide very good viewing within the defined field, and are less prone to interference from stray light sources other than the primary light source aimed perpendicular to the screen. In order to provide better viewing to a wider defined field, matte-finished screens are effective at providing a more even light distribution to an entire audience. Although hardly critical for most projection screen applications in darkened rooms, in either instance it is important that the screen absorb or transmit as little light as possible so as to assure maximum reflective image to the audience.
Reflectivity is far more critical in many other applications. For instance, displays used in electronic equipment (e.g., instrument panels, portable computer screens, liquid crystal displays (LCDs), etc.), whether relying on supplemental lights (e.g., backlight) or merely ambient light, require very good diffuse reflectant back surfaces to maximize image quality. Reflectivity is particularly critical with backlighted displays in battery powered equipment, where better reflectivity is directly related to smaller required light sources and resulting lower power demands.
Even more demanding applications for highly reflective materials are in casings used in laser construction. Since the efficiency of a laser is directly dependent upon its ability to effectively process light energy within its casing, it is critical that the casings be constructed from material that has extremely high reflectivity and excellent diffusion properties.
Perhaps the most demanding applications for reflectant materials are its use in a variety of optical test equipment, such as in reflectometers, integrating spheres, spectrophotometers, solar collectors and photovoltaic cells, etc. Highly reflective material is also used as standards in such equipment as well as serving to assure proper light handling within such equipment.
Due to the many different applications that exist for reflectant materials, it is not surprising that there are many different commercially available products with a variety of diffuse reflective properties. Until the present invention, the best material known with excellent diffuse reflectivity was that described in U.S. Pat. No. 4,912,720 and sold under the trademark SPECTRALON by Labsphere, Inc., North Sutton, N.H. This material comprises lightly packed granules of polytetrafluoroethylene that has a void volume of about 30 to 50% and is sintered into a relatively hard cohesive block so as to maintain such void volume. Using the techniques taught by U.S. Pat. No. 4,912,720, it is asserted that exceptionally high diffuse visible light reflectance characteristics can be achieved with this material, with reflectance over previously available reflectant material increasing from 97% to better than 99%. In laser casing construction, this is reported to produce an increase laser output by as much as 100% over previously available reflectant materials.
The SPECTRALON material is reported to be the most effective diffuse reflective material available today for visible light and near infrared (IR) reflectivity. As a result, in addition to laser casing construction, this material is used in a number of applications where extremely high reflectivity is required. For example, this material is considered so reflective that it serves as a standard material against which all other reflectant materials are measured in light reflectometers and other light measurement devices.
Despite the reported advantages of SPECTRALON material, it is considered quite deficient in many respects. First, this material comprises a relatively hard block of material that must be carefully carved or machined to desired shapes and dimensions. This severely limits how and where this material can be used and greatly increases the cost of using this material in many applications, especially where non-planar shapes are desired. Therefore, where a pliable material is desired in various light reflective applications, it is clear that the SPECTRALON material is not capable of supplying such a property. Furthermore, the additional machining process provides yet another source for contamination that can be detrimental to its reflective properties.
Second, the SPECTRALON material is apparently limited, both structurally and in its ability to reflect light, to a relatively thick minimum depth (i.e., a thickness of greater than 4 mm). Again, this serves to limit where and how this material can be used. Moreover, this limitation tends needlessly to increase both the amount of material required for a given application as well as the weight of the material required for such application.
Third, the SPECTRALON material is apparently relatively expensive to manufacture and purchase. These costs are only increased by the material's difficulty in processing into the final shape from the hard form (i.e., excessive amounts of material may have to be machined away and discarded during production) and its minimum thickness requirements. As a result, the SPECTRALON material is too expensive to be used in many applications that might otherwise benefit from its reflective properties.
Fourth, although SPECTRALON has high diffuse reflective properties, it is contemplated that even better performance may be possible in this regard. For instance, the SPECTRALON material has very good reflective properties for visible light up to a near IR range (i.e., from 300 to 1800 nm), the reflectivity of this material diminishes dramatically above 1800 nm. Moreover, it is believed that even better reflective performance might be possible even in the visible light range where SPECTRALON material delivers its best performance.